We plan to utilize our recently developed techniques for hybridization of the cells of lymphocyte lines to study the control of immunoglobulin synthesis in human lymphocyte-human fibroblast hybrids, and to assign the genes responsible for Heavy and Light chain synthesis to specific human chromosomes, using interspecific crosses of human lymphocytes and mouse fibroblasts. BIBLIOGRAPHIC REFERENCES: Spector, E.B., Lockridge, O., and Bloom, A.D. Citrulline metabolism in normal and citrullinemic human lymphocyte lines. Biochem. Genet. 13:471, 1975. Bloom, A.D., Nakamura, F.T., and Ohki, K. Gene dosage effect in an established lymphocyte x lymphocyte hybrid. Abstract, Amer. Soc. Human Genet., 1975.